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The effect of helium ambient gas on the expansion dynamics of plasma plume produced by ablating the LiYF 4 crystal with 355 nm laser pulses is investigated. To study the expansion dynamics, the time-resolved optical signals from the plume constituents, either neutral or ionized, were recorded as a function of various experimental parameters, including laser fluency, at different distances, d, from the target along the expansion direction of the plume and at various angles with reference to the target normal. The analysis of the plume constituents at different distances from the target was used to derive the expansion velocities of the species. Depending on the species and on d, the shifted Maxwell-Boltzmann distribution component of the fit gave the values of the expansion velocities in the range 1.0 × 10 4 m s −1 -4.0 × 10 4 m s −1 . The effect of laser fluency on the expansion velocities of the plume species was also investigated. With the exception of Y and Y + , the expansion velocities of all the plume species, increased linearly with increasing laser fluency up to ∼8 J cm −2 . Beyond this value, a saturation regime is observed. The angular distribution of the plume constituents in 1 Pa of He evidenced that all the plume species were focused along the target normal, and the FWHM of the angular distribution curve resulted reduced when compared with the vacuum.